Loop Lighting is when theres a loop-like shadow under the subject's nose (or near the corner bottom of the nose)
For Loop Lighting, the light has to be placed to one side of the person's or subject's face and positioned so you can get a loop like shaped shadow under the nose. For a really good Loop Lighting photo, have the shadow point down towards the corner of the mouth
This technique helps with oval shaped faces because of the downward angle of the loop seems to make the face look longer, and essentially raises the cheek bones
How would you do it? You have to raise the light slightly above eye level, start with the light off to the side about 45 degrees from the camera. By then, you can tweak with the amount of degrees to see where the light falls.